1. Field of the Invention
This invention relates to a process for preparing porous crystalline alumino-silicates (zeolites).
More specifically, the invention relates to a process for preparing porous crystalline alumino-silicates of high silica/alumina ratio, in an inorganic environment.
2. Description of the Prior Art
Zeolites having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of about 10 or greater are notably characterized by high thermal stability, which allows them considerable variety of application.
Such zeolites are mainly characterised, at least in the case of known structures, by structural sub-units composed of rings of 5 tetrahedra, such as the zeolites of the mordenite family, which includes ferrierite, dachiardite, epistilbite and bikitaite. On the other hand by means of the template action of organic substances, preferably tetraalkylammonium derivatives, but also alcohols, ethers and amines, it is possible to obtain special zeolites. Of this family, the most known is ZSM 5, which also contains rings of 5 tetrahedra. Analogous zeolite structures have been obtained by varying the nature of the template (Nu 1, ZSM 35, ASM 11, ZSM 12 etc.).
Purely siliceous structures have been obtained by action of the tetraalkylammonium template (silicalite 1, silicalite 2 and analogues) where the use of a purely inorganic system would have led to the formation of quartz or metastable crystalline silicas, such as silica .chi., which then evolve into quartz. An attempt to substantially reduce the quantity of tetraalkylammonium cation in the synthesis of ZSM 5 and ZSM 11 zeolites has been made using mixtures of ammonium hydroxide and/or alcohols in the presence of nuclei of the zeolite which it is required to produce.
The need to use crystallization nuclei is, however, a process complication, in that such nuclei must be chosen carefully with regard to their dimensions and quantity.
In European patent application No. 30811 a process has been proposed for the synthesis of zeolite alumino-silicates with a molar silica/alumina ratio greater than 12 without using organic compounds such as alcohols, tetraalkylammonium ions or alkylene oxides. Ammonia, ammonium hydroxides or ammonium salts are used in order to obtain the formation of the required zeolite.
Finally, U.S. Pat. No. 4,257,885 describes a new synthetic zeolite prepared in the absence of organic substances or ammoniacal substances, using colloidal silica as the silicon source.